The present invention relates generally to the field of marine electromagnetic surveying. More particularly, in one or more embodiments, this invention relates to methods and systems for conditioning electrodes while deployed in the sea, for example, with a marine electromagnetic survey system.
One technique for marine electromagnetic surveying involves towing an energy source at a selected depth in a body of water. One or more survey cables also may be towed in the water at selected depths. The survey cables are essentially long cables having electromagnetic sensors disposed thereon at spaced apart locations. The energy source and survey cables may be positioned in the water by attached equipment, such as deflectors and position control devices. Actuation of the energy source emits an energy field into the body of water. The energy field interacts with the rock formations below the water bottom. The change in the energy field due to the interaction with the subterranean rock formation is detected by the electromagnetic sensors and used to infer certain properties of the subsurface rock, such as structure, mineral composition and fluid content, thereby providing information useful in the recovery of hydrocarbons. In addition to towed survey cables, electromagnetic survey systems may also use sensors that are at a fixed location with respect to the energy source, which may include attachment of electromagnetic sensors on one or more cables positioned on the water bottom or attachment of the electromagnetic sensors to one or subsurface acquisition nodes, for example.
Electrodes may be used in one or more of the components that are used in electromagnetic survey systems. For example, electromagnetic sources and electromagnetic sensors may each include a pair of electrodes. When deployed for an electromagnetic survey, these electrodes can be exposed to seawater, which may undesirably interact with the surface of the electrodes. For example, because a potential difference is typically applied across the electrodes during use, the surface of the electrodes can react with the seawater causing undesirable deposits that can degrade electrode performance. In addition, electrode performance can also be degraded by algae and other biological contaminations or growths that may occur on the surface of the electrodes. In some instances, the degradation of electrode performance caused by these undesirable interactions can limit the operational life of the electrodes used in the marine electromagnetic survey systems, as the interactions may result in decreased accuracy of measurements from sensor electrodes, for example. The current technique for restoring performance to the electrodes when these undesirable interactions have undesirably degraded their performance is to retrieve the electrodes and install replacement electrodes above the surface of the water. Drawbacks to this technique for restoring performance include the undesirable impact on the operational efficiency and safety of the survey systems caused by the time, effort, and expense associated with retrieval and replacement of the electrodes.
Accordingly, there is a need for improved methods and systems for restoration of electrode performance in marine electromagnetic survey systems.